Gary Fire Department Battalion Chief Donald Williamson says the two eastbound engines collided early Sunday morning. He says the accident was apparently caused by a faulty switch gear.
Dozens of train cars went off the tracks into the Miller neighborhood.
According to the Chicago Tribune, firefighters had to extricate two people, including a crew member who had a broken leg.
Williamson says the train wasn't hauling hazardous materials or tankers and there are no leaks. He said emergency crews responded as if there were hazardous materials as a precaution.
Residents within a square mile of the accident were asked to stay inside their homes until crews could clear the scene.
In a phone interview, a spokesman for CSX said they're working as fast as they can to clear the high volume tracks.
"We're working with the authorities in Gary as to the impact on the residents in the area," said spokesman Gary Sease. "Thankfully, there are no threats to public health or safety from the derailment, but we will have heavy equipment in the area all night.
"We're working with the residents to make sure they're not inconvenienced any more than they already have been and that they're in a comfortable place this evening."
The collision did not affect the South Shore electric commuter train. Those have been running normally all day.
(The Associated Press contributed to this report.)